Many of types of prior battery chargers exists wherein the primary objectives are not only to charge the battery but to accomplish this in a minimum amount of time without damaging the battery. Some prior art battery chargers implement an initial rapid battery charging at a high current magnitude. When a battery condition is sensed which would indicate potential damage to the battery may occur if this rapid initial charging current is maintained, the rapid charging current is terminated and a subsequent charging current is implemented. In some cases this subsequent charging current is just a maintenance charging current which compensates for any leakage or discharge from the battery. In other cases, the subsequent charging current can be a top off interim charging current which is then followed, after a predetermined time period, with a maintenance charging current.
The prior battery charging apparatus use a variety of techniques to terminate the initial charging current and implement the subsequent charging current. However, changes to the subsequent charging current in prior art systems are typically implemented after some fixed time period and/or in response to comparing battery voltage to some predetermined fixed voltage threshold. Because of this, prior systems are inefficient in that they do not account for variations in the maximum voltage that a battery can be charged to due to the age or other conditions of the battery. Thus, prior systems may attempt to over or under charge a battery because they assume that the battery will always want to be charged to its nominal battery voltage. The present invention attempts to overcome this by taking into account that the voltage that a battery wants to be charged to for full charging may vary with the age or condition of the battery as well as manufacturing tolerances. Thus to account for this, the present embodiment takes into account the variations in maximum full charge battery voltage which occur during the lifetime of a battery.